A further improvement in design can be seen in quests, both in overarching structure and individual quest design. The main quests are no longer of a linear, one-after-the-other sort, but rather the main quest givers will give you a number of important jobs, and once you have done a couple of them you can move on. Most players will do all the quests regardless, but this does mean that you can do these quests in whichever order you like, or leave quests undone if you don't feel like doing them for whatever reason. In particular, some of the more morally questionable quests at the end are eminently skippable, and I definitely appreciated how the game gave me the option of ignoring half of them if I so chose.
Avernum's open-world nature is still limited by these main quests, and you will have to progress through them to open up new areas. But interestingly enough, Avernum 6 will at times tell you to go to a specific place directly, it being very dangerous for you not to do so. And while it is indeed dangerous, the game won't force you to do so, and you can wander around more freely than you might expect. The game's open-world design is pretty good here, and even when you're doing quests out of order you usually won't break them.